The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to the techniques used for stimulating hydrocarbon-bearing formations—i.e., to increase the production of oil/gas from the formation and more particularly, to a process for optimizing fluids for and monitoring fluid rheological performance during fracture stimulation treatments.
Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation and thus causing a pressure gradient that forces the fluid to flow from the reservoir to the well. Often, a well production is limited by poor permeability either due to naturally tight formations or due to formation damages typically arising from prior well treatment, such as drilling, cleaning etc.
To increase the net permeability of a reservoir, it is common to perform a well stimulation. A common stimulation technique is hydraulically fracturing a formation penetrated by a wellbore. Hydraulic fracturing typically consists of pumping a proppant-free viscous fluid, or pad, usually water with some fluid additives to generate high viscosity, into a well faster than the fluid can escape into the formation so that the pressure rises and the rock breaks, creating artificial fractures and/or enlarging existing fractures. Then, proppant particles are added to the fluid to form a slurry that is pumped into the fracture to prevent it from closing when the pumping pressure is released. The proppant suspension and transport ability of the treatment base fluid traditionally depends on the type and concentration of viscosifying agent added.
Modern fracturing technology relies on fluids that exhibit flow behavior that changes over the course of a fracturing treatment. A fracturing fluid must be viscous enough to carry the proppant through the perforations and through the fracture, and to minimize fluid loss to the formation. On the other hand, the fluid should ideally be thinner in the tubing to limit horsepower requirements and to minimize shear degradation. To facilitate an efficient clean-up, its viscosity must be reduced to an absolute minimum after the treatment is over, thus ensuring optimum fracture conductivity and well productivity. With modern fracturing technology, a single fluid can meet all of these requirements, but a successful fracturing treatment requires a careful fluid design, i.e. the fluid composition should be chosen based on formation temperature and pressure, pumping rate, pumping time, completion capacity, water quality etc. The fluid design for a fracturing treatment is traditionally determined based on both experience and laboratory testing, whereas modeling has previously only played a minor role or no role at all.
As stated above, for fracturing fluid designs, fluid compositions and breaker schedules for a fracturing job is most commonly determined by lab rheology measurements and/or experience. Models are not solely used in the determination of the gel loading, crosslinker concentration, breaker schedule etc. In some instances, fracturing simulators employing simple models such as the power law model and the Cross model are used to represent the fluid rheology. These models are regressed to the rheology data for the actual fluid being pumped, so experimental data is needed. None of these models account for the live chemistry of fracturing fluids, i.e., the fact that crosslinks are dynamically formed and broken, as well as polymer linkages that are broken by thermal degradation or degradation by oxidizers.
Further, the use of techniques to quality assure, quality control (QA/QC) the fracturing fluid real-time are not known or used at this time. QA/QC of fluids typically are conducted on a time delayed sampling—testing basis.
Therefore, the need exists for methods that can reduce the number of laboratory experiments needed, as well as techniques which enable real-time QA/QC of fracturing fluids, so that the treatment may be adjusted if needed. Techniques which achieve the above would be highly desirable, and these needs are met at least in part by the following invention.